Tis invention relates to an improvement in a dither circuit employed for improving linearity in analog-to-digital (A/D) or digital-to-analog (D/A) conversion of a signal and, more particularly, to a dither circuit capable of reducing an error in a small signal and also reducing an adverse effect of error in accuracy in addition and subtraction in analog thereby to facilitate practical application of dither.
In A/D or D/A conversion, quantizing noise is inherently generated. As is well known, the quantizing noise becomes a white noise which is uncorrelated with a singal to be converted if the signal is of a large level whereas the quanitizing noise becomes distortion which is greatly correlated with the signal if the signal is of a relatively small level and this noise adversely affects the quality of tone when the signal is sounded in an audio apparatus.
For overcoming this problem, employment of a dither circuit with an A/D or D/A converter is proposed. A dither circuit superposes dither (noise signal) on an input signal before A/D or D/A conversion and subtracts dither from an output signal after the conversion (in a case of a subtraction type dither circuit). By way of example, in the case of A/D conversion, as shown in FIG. 2, dither (digital signal) generated by a noise generator 10 is converted to an analog signal by a D/A converter 12 and the converted dither is superposed on an input analog signal by an adder 14. After the input signal superposed with dither is A/D-converted by an A/D converter 16, dither is subtracted from the output of the A/D converter 16 by a subtractor 18 whereby a digital signal corresponding to the input analog signal is derived.
In a case of D/A conversion, as shown in FIG. 3, dither (digital signal) generated by a noise generator 10 is superposed on an input digital signal by an adder 14, the input signal superposed with dither is D/A-converted by a D/A converter 20, and dither D/A converted by a D/A converter 12 is subtracted from the output of the D/A converter 20 by a subtractor 18 whereby an analog signal corresponding to the input digital signal is derived.
Dither in a classical sense of the term is a technique used for reducing a quantizing noise accompanying a quantizing error and there are two types of dither, i.e., dither which imparts simply an amplitude noise for LSB (least significant bit) one bit incidentally, (the noise level naturally increases) and dither according to which noise is superposed on an input signal and its noise is subtracted after A/D or D/A conversion to prevent increase in the noise level.
If an A/D or D/A converter is ideally manufactured, the amplitude of noise used for dither can be .+-.1/2 LSB. In practice, however, an amplitude which is sufficiently larger than this value has proved more effective. This is because a converter of sixteen bits, for example, has not necessarily an accuracy of sixteen bits but the accuracy generally drops to fourteen to fifteen bits. Dither of a large amplitude is effective for such converter because a conversion error is dispersed by using such dither of a large amplitude as compared with a case where ordinary dither is used so that monotonousness of the converter is overcome and its linearity is improved.
If, as shown in FIG. 4, there exists a conversion error in an A/D converter, an output digital signal S' is distorted against an input analog signal S of a small amplitude. By superposing dither D of a large amplitude on the input signal S, the conversion is made with a broad range B instead of a range A which is a case where no dither is used (dither is subtracted after the conversion) so that the conversion error is dispersed and linearity is thereby improved. Accordingly, an A/D or D/A converter of less accuracy requires dither of a larger amplitude.
Factors determining accuracy of A/D and D/A converters will now be considered.
Since an A/D converter is generally composed of a D/A converter and a feedback circuit, its accuracy is greatly influenced by the D/A converter.
Accuracy of a D/A converter is generally attained by resistance trimming. As a result, the severest accuracy is required for resistance corresponding to MSB (most significant bit) at which the largest weighting is placed. Consequently, most D/A converters generate the largest error when MSB is switched. Besides, since the accuracy of the D/A converter is attained one by one by trimming, the conversion error becomes irregular with its value differing one converter from another.
The same applies to an A/D converter which uses a D/A converter. Moreover, since the D/A converter is operated the number of times corresponding to its bit number for a single A/D conversion, it is operated at such a high speed that its accuracy in conversion is generally decreased even further than a D/A converter so that necessity for dither is even higher than in a D/A converter.
Particularly in a PCM audio device such as a Compact Disc Digital Audio System and a digital audio tape recorder, MSB is used as a sign bit representing the polarity of a signal and is switched at zero crossing of an input signal so that influence of an error occurring at MSB over the quality of a tone produced is extremely great.
Since the purpose of dither is improvement in characteristics of a small signal, dither will become meaningless unless it is effective against such conversion error occurring at MSB. As described above, in view of the facts that there is a high probability that a conversion error at MSB is the largest of errors occurring in A/D and D/A converters and that there is irregularity in the accuracy of converters, dither of a substantially large amplitude must be applied.
Use of dither of such large amplitude, however, is extremely difficult in practical application to converters actually manufactured. Because, dither is added and subtracted in prior and posterior stages of a converter with at least either of one of the addition and subtraction being effected in analog and accuracy in such analog addition and subtraction becomes a problem. Namely, since a trimming error tends to occur in dither used for addition or subtraction which is produced by D/A-converting an output signal of a noise generator, an accurate subtraction becomes difficult. For this reason, it is difficult to realize dither exceeding a certain value (i.e., a certain bit number) in view of this irregularity in conversion error, even though dither of a large amplitude is generally desirable.
In actually manufacturing a converter, it is desirable that only a rough adjustment of a product suffice but, this rough adjustment poses problems in an absolute accuracy in the analog addition and subtraction. Accordingly, it is difficult to realize a large amplitude dither in practical use.
For overcoming this problem, band concentration dither has been proposed. According to this proposal, noise of a band which is substantially inaudible (generally a high frequency band) is used as dither. This method, however, has the problem that construction of a generator producing such noise becomes complicated.
It is, therefore, an object of the invention to provide a dither circuit overcoming these problems and being capable of reducing error in a small signal and also reducing influence of errors such as error in addition or subtraction of dither in analog.